Multi-blade centrifugal fans, called sirocco fans, include an impeller composed of a disc-shaped hub whose center is convex on the intake side, a plurality of blades (also called blades, vanes, or the like) arranged radially on the periphery of the hub, and an annular shroud disposed at the opposite ends of the blades from the hub, and a scroll-shaped fan casing in which the impeller is rotatably supported. For a typical multi-blade centrifugal fan, the shape of the blades in a cross-section perpendicular to the rotating shaft of the impeller is substantially uniform in the axial direction, that is, two-dimensional. This is so that the impeller can be formed by plastic molding at relatively low cost.
The multi-blade centrifugal fan deflects a flow taken in in the direction along the rotating shaft to a centrifugal direction perpendicular to the rotating shaft through the impeller and blows it from the periphery of the impeller into the casing. This causes a problem in that it is difficult to fully utilize the entirety of the blades because the flow is insufficiently deflected on the shroud side, which is closer to an intake port, and also less easily reaches the vicinity of the hub, with the result that the flow concentrates at a position slightly closer to the hub than the center of the blades in the spanwise direction. In addition, because the blades have a uniform cross-sectional shape despite the flow state varying in the direction along the rotating shaft, the blade shape does not match the flow, which results in decreased efficiency and airflow disturbance, thus leading to increased fan input power and noise.
Various proposals have thus been made for improved fan efficiency and reduced noise. PTL 1 discloses a multi-blade centrifugal fan including blades curved in a concave shape on the pressure side and satisfying β2<β3, where β2 is a middle angle between a tangent to a circle whose radius is a line segment joining the middle point in the middle portion between the inner and outer ends of each blade and the center of the fan and the surface of the blade at the middle point, and β3 is an exit angle between a tangent to a circle whose radius is a line segment joining the exit point of the outer end of each blade and the center of the fan and the blade surface at the exit point, which is intended to relatively increase the static pressure component for reduced noise and increased fan efficiency.
In addition, PTL 2 discloses a multi-blade centrifugal fan including blades having a tapered portion formed at least at one end of an inner edge (leading edge) thereof in the axial direction such that the inner diameter thereof increases from the other end to the one end in the axial direction, the tapered portion being located forward in the rotational direction and having an entrance angle of 55° to 76° for increased work of the impeller, improved efficiency, and reduced noise. In addition, PTL 3 discloses a multi-blade centrifugal fan including blades curved in a concave shape on the pressure side such that they are backward-swept near the leading edge thereof and are forward-swept near the trailing edge thereof, wherein the sum of an entrance angle β1 and an angle β′2 is set to less than 80° to reduce the noise level without a decrease in the volume of air, where β1 is the entrance angle of the blades, β2 is the exit angle of the blades, and β′2 is the difference obtained by subtracting the exit angle β2 from an angle of 180°.